A component made of piezo elements stacked one above another is designated a (piezo) stack. The function of a piezo element is based on the deformation of a piezo-ceramic material, such as lead-zirconate-titanate, and the action of an electric field. If an electric voltage is applied to a piezo element, then the latter expands in the direction perpendicular to the electric field generated by the electric voltage. A single piezo element has a relatively small actuating travel, for which reason, for a larger actuating travel, use is made of a piezo stack made of a plurality of piezo-electric layers following one another alternately and what are known as inner electrode layers.
The inner electrode layers are normally alternately connected electrically to outer electrodes arranged on opposite outer surfaces of the piezo stack. The inner electrode layers, which are electrically connected to one of the two outer electrodes, are therefore led as far as the outer side on which this outer electrode is arranged for the electrical connection to the outer electrode. In order that the inner electrode layers are insulated electrically from the other outer electrode, the inner electrode layers do not reach as far as the outer side of the piezo stack on which the further outer electrode is arranged. In these areas, the inner electrode layers are set back from the outer side. This is achieved, for example, in that the piezo stack in these areas is provided with slots filled with silicone resin, plastic or varnish.
As a result of the set-back inner electrode layers, the result in the piezo-electric layers associated with these areas is what are known as inactive zones, which, when an electric voltage is applied to the outer electrode layers and inner electrode layers, are penetrated by a reduced electric field strength and therefore, when an electric voltage is applied, expand less highly than the other active zones, as they are known, of the piezo-electric layers. This leads to mechanical stresses, in particular in the inactive zones and the edge regions associated with the inactive zones, and can lead to what are known as poling cracks in the inactive and active zones of the piezo-electric layers and in the outer electrodes. The danger of poling cracks is therefore all the greater, the larger the inactive zones.
In order to be able to provide a piezo actuator with a piezo stack in which the smallest possible inactive zones are formed, DE 10 2006 011 293 A1 proposes a method which is based on further processing a fully active green stack.
A fully active green stack (or fully active piezo stack) designates a green stack (piezo stack) formed from a plurality of green films (piezo-electric layers) and electrically conductive layers arranged between the former, in which the electrically conductive layers form the inner electrode layers of the stack and are formed continuously as far as the outer sides of the stack. Such a fully active green stack (piezo stack) accordingly has no inactive zones but only active zones, i.e. a green film (piezo-electric layer) arranged between two inner electrode layers and belonging to the fully active green stack (piezo stack) is completely covered on one side thereof by the one inner electrode layer and on the other side thereof by the other inner electrode layer. One advantage of such a fully active piezo stack is its relatively simple production using known process steps.
The inner electrode layers are provided to be connected electrically alternately to outer electrodes to be arranged on the outer side of a piezo stack produced from the green stack. However, in order that the inner electrode layers are electrically connected only to the outer electrode provided therefor and remain electrically insulated from the other outer electrode, the areas of the outer side on which the further outer electrode is later arranged are provided with trenches. By means of the trenches, the inner electrode layers in these areas are set back from the respective outer side of the fully active green stack, which means that extremely small inactive zones are produced in these areas. In order that, during the later application of the outer electrodes, the inner electrode layers are also electrically connected only to the outer electrode provided, the trenches are filled with the electrically insulating slurry, a liquid mixture of ceramic powder and binders, before the application of the outer electrodes.
With the method proposed in DE 10 2006 011 293 A1, the trenches and thus the inactive zones of the piezo stack provided with the trenches can be dimensioned such that they are no larger than is absolutely necessary for good electrical insulation of the inner electrode layers with respect to the corresponding outer electrode. As a result, the inactive zones of this piezo stack can be designed to be as small as possible, which means that the danger of producing poling cracks is reduced.
In order to produce the trenches, DE 10 2006 011 293 A1 proposes using a laser. One disadvantage of this procedure is that, as a result of lasering the trenches, the organics in the green stack burn in these areas. In the process, the ceramic can be partly broken down, forming refractory primary oxides such as ZrO2 and TiO2. The material burned away must subsequently be removed completely from the trenches in order to provide pore-free and crack-free filling of the trenches. This is possible only with considerable effort, the danger of damaging the piezo stack at the same time being high.
In order to introduce the slurry into the trenches, DE 10 2006 011 293 A1 likewise proposes falling back on the laser method used to create the trenches. One disadvantage of this procedure is that the method is relatively time-consuming and costly, since the production of the trenches by the laser has to be carried out sequentially.